Aquat. Sci. 67 (2005) 350– 362 1015-1621/05/030350-13 DOI 10.1007/s00027-005-0777-2 © Eawag, Dübendorf, 2005

Aquatic Sciences

Research Article

Water quality and phytoplankton communities in Lake Qarun (Egypt) Adel A. Fathi 1, * and Roger J. Flower 2 1 2

Botany Department, Faculty of Science, El-Minia University, El-Minia, Egypt Environmental Change Research Centre, University College London, 26 Bedford Way, London WC1H OAP, United Kingdom; [email protected]

Received: 7 September 2004; revised manuscript accepted: 2 March 2005 Abstract. Lake Qarun is a closed saline lake in the northern part of El-Fayoum Depression (Middle Egypt) at the margin of the Great Western Desert. It is almost entirely sustained by inflow from the Nile River and, during the 20th century, lake water salinity has increased strongly. Physico-chemical characteristics and phytoplankton periodicity in the lake were monitored during 2001. All the water quality variables measured showed considerable seasonal variation, and quantitative and qualitative differences in phytoplankton communities were recorded. The maximum crop density was in August, whereas lowest values occurred in December. Highest crop densities coincided with a high abundance of Bacillariophyceae. The Bacillariophyceae were most diverse with 23 species,

then Chlorophyceae with 16, Cyanophyceae with 8, and Chrysphyceae and Dinophyceae with one species each. Despite being a saline inland lake, the open-water phytoplankton communities were composed of some marine/ brackish forms but mainly of freshwater communities tolerant to high salinity. The identified phytoplankton species indicate a tendency towards eutrophy but total crop densities were relatively low compared with eutrophic lakes elsewhere. Light limitation by suspended solids as well as hydrological related factors are believed responsible for the relatively low phytoplankton abundance. The lake appears to be ecologically unstable and careful limnological monitoring is recommended.

Key words. Water chemistry; phytoplankton.

Introduction Inland lakes are unusual in dry climate zones and Lake Qarun is the only significant natural lake in Middle Egypt. It occupies part of the basin of ancient Lake Mories, an immense freshwater palaeo-lake that persisted until the mid Holocene (Hassan, 1986). The basin is fed by the Nile River and the lake occupies the lowest level of the El-Fayoum depression, an extensive and fertile oasis situated at the edge of the Great Western Desert of Egypt and Libya (Anonymous, 1995). Comparing historical lake data (Ball, 1939) with modern biological and chemical characteris* Corresponding author e-mail: [email protected] Published Online First: August 22, 2005

tics indicates that Lake Qarun has experienced major environmental changes in the recent past. Lake Qarun now has the salinity of seawater, but the presence of freshwater diatomite deposits near the northeastern shore (Aleem, 1985) shows that the salt content of lake water has increased markedly over time. Gradually increasing salinity has accelerated since ~1900s (Ball, 1939), reached 32–36 g L in 1975/76 (Boraey, 1980), and caused a profound effect on lake fauna and flora. Contemporary phytoplankton now contain some euhalobous/ mesohalobous species that typically occur in coastal environments (Soliman, 1989). Diatoms are seasonally common in phytoplankton (Kobbia et al., 1992) and zooplankton are characterized by copepods, rotifers, tintinnids and ostracods (Sabae and Rabeh, 2000).

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Lake Qarun is environmentally important; it is a wetland of international importance for waterbirds and its used to support a major fishery. Salinity has become unfavorable to most of the freshwater fish that originally occurred in the lake (Boraey, 1980). All the original freshwater fish are gone except Tilapia zillii. To compensate, the lake was stocked with Mediterranean fish since the 1920s (El-Maghraby and Dowidar, 1969). During the 20th century the lake has increasingly become ecologically unstable and several publications report on the lake’s changing environment. Physical and chemical variables are documented by Naguib (1958), Meshal (1973), Ishak and Abdel-Malek (1980), and Anon (1997). Sabae (1993; 1996), Sabae and Rabeh (2000), Wimpenny and Titterington (1936), Girgis (1959), Khalil (1978), Shaaban et al. (1985), Abdel-Malek and Ishak (1980), Abdel-Moniem (1991), Gad (1992), AbdelMoniem (2001) and recently Mansour and Sidky (2003) investigated the plankton communities. Abdel-Malek and Ishak (1980) and Fishar (1993) studied the benthic fauna. El-Shabrawy and Taha (1999) examined the effect of zooplankton grazing pressure on phytoplankton assemblages. Lake fisheries was studied by Wimpenny and Faouzi (1935), Faouzi (1936), Wimpenny (1936), El-Zarka (1963), El-Zarka and El-Sadafy (1967), Boraey (1974), Abdel-Malek (1980; 1982), Ishak et al. (1982), Sweilum (1989), Mosaad (1990), Gabr (1998) and ElShabrawy and Fishar (1999). Long-term changes in the phytoplankton of Lake Qarun during the last 30 y showed a severe decrease in species. Nosseir and Abou El-Kheir (1971) recorded 85 species in one season. In 1989, Abdel-Moniem, (1991) recorded 119 taxa in a seasonal investigation, Kobbia et al. (1992) recorded 64 taxa and recently 50 taxa were recorded in a seasonal investigation by Abdel-Moniem (2001). Diversity decline may reflect the impact of pollution loading or other environmental changes such as the increasing salinity. Rabeh (2001) reported a notable increase in bacterial numbers in lake water that may indicate adverse ecological change. However, most research on Lake Qarun occurred from the 1960–1980s and studies were short term with little detail on the lake ecosystem. Lake Qarun can be considered a natural reservoir of drainage water collected from irrigated cultivated lands in El-Fayoum that have been highly modified since the early 20th century. Preceding research comprised investigations and surveys that are not directly applicable to conditions in the 21st century. Accordingly, an investigation of phytoplankton and water quality characteristics of the lake during 2001 was undertaken. This study was designed to examine some of the seasonal influences of water quality on phytoplankton and to give base-line conditions for assessment of future change. Results of routine monthly sampling of water chemistry and phytoplankton over 12 mo. are reported.

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Site description and environmental context Lake Qarun is a closed saline lake in the northern part of El-Fayoum Depression (Middle Egypt, ~80 km southwest of Cairo, at the margin of the Nile Valley (Fig. 1). It is centered around 29°30¢N, 30°40¢E. The length of the lake from east to west is about 40 km, and the breadth at its widest point is about 6.7 km. The lake has a surface area of 243 km2 and a volume of 924 million m3 at 43 m below sea level (Anonymous, 1995). The deepest point (~8.3 m) is northwest of the island. Non-irrigated northern shores of the lake are virtually devoid of vegetation and mark the beginning of the Western Egyptian Desert. The lake has no connection to the sea, being located 320 km south of the Mediterranean coast of Egypt, and is sustained directly by the Nile River via the Bahr Yussef canal. The lake has received predominantly agriculture wastewater since the beginning of the 18th century (Anonymous, 1995). The total water draining annually into the lake is about 395 million cubic meters (data supplied by the Irrigation Department, El-Fayoum). Average total dissolved solids are relatively low in surface inflows (~1.25 g L–1). Approximately 4% of the drainage water is untreated sewage. In addition to water diversion for the Wadi El-Ryan Project in the 1960s, almost continuous change in general water management have occurred during the last 100 y and these are set in a context of 4,500 y of hydrological modification (Hassan, 1986). Water level variations are summarized in Table 1, and by the 20th century levels had declined by more than 60 m since the Pharonic period. A recent increase in water level suggests in other sources of water besides surface drainage is discharged into the lake. This increase can be attributed to seepage from adjacent groundwater aquifers and from the Wadi El-Rayan Depression (El-Sayed and Guindy, 1999; Mansour and Sidky, 2003). Further, the accumulation of silt fills the lake Table 1. Historical changes in water level of Lake Qarun (Anonymous 1995, El-Sayed and Guindy, 1999). Time Ancient state Twelfth Dynasty Ptolematic Dynasty Romans Salah El Din Mohammed Ali

Levels in meters 5000-3063 BC 3021-2808 BC 330-30 BC 381-640 1171-1250 1805 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 1999

+ 20.00 + 15.00 00.00 – 20.00 – 30.00 – 40.00 – 44.45 – 44.50 – 44.33 – 44.80 – 44.27 – 44.25 – 44.05 – 43.95 – 43.55 – 43.30 – 43.25

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Phytoplankton and water quality in Lake Qarum (Egypt)

Figure 1. Location map of the study region. Four sampling sites in Lake Qarun are shown.

by ~1 cm annually (Anonymous,1995), and this process encourages water overflow that damage on surrounding agricultural land and buildings through inundation. Salinity of Lake Qarun increased throughout the 20th century (Fig. 2), being around 14‰ in the early 1900s (Ball, 1939). Mean salinity reached an average ~38 ‰ in the 1980s but, since then, has stabilized. Seasonally, soluble salts in Lake Qarun increase during summer and this is probably due to evaporation and changes in the inflow regime. Being located in an arid area with intensive evaporation (~7 mm h–1), freshwater input does not compensate for seasonal evaporation and salinity increases. The salinity of the lake is not homogenous, being lower near the mouth of the inflows that drain the lakes southern landscape.

Largely because of increased salinity, the aquatic biota have changed markedly (Anonymous, 1995). The lake fishery was formerly characterized by significant stocks of Lates niloticus (Nile perch), Clarias anguillaris (cat fish), Labeo niloticus, Barbus bynni, Anguilla vulgaris (Common Eel), Tilapia zillii, and Oreachromis niloticus. As a result of the disappearance of most of these species, the commercial catch decreased from 4000 tons in 1920 to an average of 1000 tons in subsequent years. To compensate, the lake was restocked with fish of marine origin, i.e. Mullet (Mugil cephalus, Liza ramada; Liza saliens; Liza aurata), Atherina mochon, Anguilla vulgaris, Solea aegyptiaca and marine prawns (Ishak et al., 1982; El-Shabrawy and Fishar, 1999). Despite the water quality and biological changes in recent decades, the lake remains internationally important for birds and large concentrations of Black-necked Grebe (Podiceps nigricollis), Shovelers (Anas clypeata) and Slender-billed Gull (Larus genei) still use the site (Tharwat, 1997).

Materials and methods

Figure 2. Changes in salinity in Lake Qarun during the period from 1901–2000 (Soliman, 1990; Anonymous, 1995; El-Sayed and Guindy, 1999).

Physico-chemical characteristics Surface water samples were collected monthly, from January to December 2001, at four sites within the lake (Fig. 1). Sites were distributed around the lake to give good spatial representation of water quality. Sampling

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was done from a small boat in open water locations, each several 100 m off-shore. Temperature, pH, secchi disc depth and conductivity were measured at each location. Dissolved oxygen was measured according to the Winkler method (Strickland and Parsons, 1972). Total alkalinity, phosphate-P, nitrate-N, chloride, sulfate, silicate and major cations were determined according to Adams (1991). Sodium and potassium concentrations were determined photometrically by flame emission according to Golterman and Clymo (1971). Results were calculated as mean values of triplicate measurements made on each water sample from each of the four sampling stations.

Analysis of phytoplankton For phytoplankton analysis, one liter water samples were fixed in the field with acidified Lugol’s solution. In the laboratory, samples were allowed to settle for at least 36 h, and then the supernatant was carefully removed and the remaining volume adjusted to a fixed volume (100 ml). This sample was kept at 4 °C until analysis. Phytoplankton counts were done using a phase contrast Carl Zeiss microscope (Jena Med2) at 100¥ and 40¥ magnification, following the Utermöhl technique (Utermöhl, 1985). Diatoms were examined in permanent preparations after cleaning in hydrogen peroxide. Permanent slides used for diatom examination were prepared mainly according to Barber and Haworth (1981). For algal counting, the simplified methods described by Willén (1976) and Hobro and Willén (1977) were followed. Counts of phytoplanktonic algae (unicellular, colonial or filamentous) were expressed as cells per liter. Cell numbers were calculated as mean values of triplicate measurements at each of the four sampling stations. Algal taxa were identified according to standard references, including Smith (1950), Fott (1972), Bourrelly (1981), Komarek and Fott (1983), Prescott (1987), and Krammer and Lange-Bertalot (1986, 1988, 1991 a and b). Brillouin’s index (H) as described by Pielou (1966) was used for quantitative analysis of species diversity.

Results Physico-chemical characteristics Average water temperature reached a maximum in August and September (30 °C) and a minimum value (16 °C) was recorded in February (Fig. 3a). The pH was always alkaline (Fig. 3b) and fluctuated between 8.2 (September and October) and 9.5 (February). Water transparency (Fig. 3c) was greatest during winter and reached a maximum in February (11.5 cm), while minimum values were recorded in July and August (~5 cm). Dissolved oxygen content of lake water tended to be highest in summer and

Figure 3. Monthly variations in temperature (a), pH (b), transparency (c), dissolved oxygen (d), and total soluble salt (e), in water of Lake Qarun during 2001.

lowest in winter (Fig. 3d). Total soluble salts (salinity) were relatively high in summer (mean, 43.9 g L–1) and decreased to minimum levels during winter months (mean, 26.4 g L–1). It is noteworthy that some dissolved salts in Lake Qarun have higher concentrations than in sea-water (Table 3). Major anions, sulphate, nitrate-N, phosphate-P, total alkalinity and silicate-Si varied seasonally (Fig. 4 a–e). Values of divalent (calcium and magnesium) and monovalent cations (sodium and potassium) were relatively high in all months but with some seasonal fluctuations. Monthly levels of calcium and magnesium fluctuated within the range of 221–526 mg L–1 and 87–200 mg L–1,

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Phytoplankton and water quality in Lake Qarum (Egypt)

Table 2. The mean values from 4 sampling stations of phytopnakton taxa recorded in Lake Qarun during 2001. J Chlorophyceae Actinastrum gracillimum Smith Ankistrodesmus falcatus (Corda) Ralfs Ankistrodesmus gracilis (Reinsch) Korshlkov Botrycoccus braunii (Kützing) Chlorella vulgaris (Beyerinck) Chlorella pyrenidosa Chick Chlorococcum humicola (Näg.) Ralfs Chlorogonium maximum Skuja Closterium acutum Brébisson Enteromorpha inteetinalls Grev. Pediastrum boryamum (Turp.) Menegh Planktosphaeria gelatinosa G. M. Smith Scenedesmus acuminatus (Lag.) Chodat Scenedesmus arvemensis Chodat Scenedesmus quadriquda (Turp.) Bréb. Staurastrum sp. Bacillariophyceae Amphora paludosa W. Sm. Anomoeoneis sphaerophora Asterionella sp. Bacillaria paradoxa Gemelin Cocconeis placentula Ehrenberg Cyclotella caspia Grun. Cyclotella meneghiniana Kützing Cymbella sp. Diatoma vulgaris var brevis Grun. Ellerbeckia arenaria R. M. Crawford Fragilaria construens Ehrenberg Fragilaria capucina Desmaziéres Gyrosigma acuminatum Kützing Melosira granulata Ralfs Navicula sp. Nitzschia closterium Ehernberg Pinnularia sp. Pleurosigma sp. Stephanodiscus invisitatus Hom & Hellerm Stephanodiscus sp. Synedra acus Kützing Synedra ulna (Ehrenberg) Tabellaria sp

F

M

1

A

M

J

1

1

1

J

A

S

1

1

1 1

1 1 2 1 1 1 1 1 1 1 1

1

1

1

2

2 2 1 2 2 2 2 2 2 2 1 1 2 2 2 1 2 1 1 1 1 1

1 1 2 1

1 1 2 1 1 1 1 1

1 2 1 2 1 1 1 1 1

1 2

1 1

2

2

O

N

D

1 1

1

1

1

1

1 1

1

1

1 2

1

2

2

1

1

1

1

2 2

2 2 1

1 1

1 1

2

2 2 3 2 2 1 3 1 1 1 2 2 1 1 1 1 1 1

2 2 3 2 1 2 3 1 1 2 2 1 1 1 1 1 1 1 1

3 2 3 1 1 1 1 2

3 2 3 1 1 2 1 2

1 3 2 1 1 1 1

1 2 1 1 1 1 1

3 2 4 1 1 1 1 2 1 1 2 1 1 1 1 1

1 1

1

1

2

1

2

1 1 1

1

1 1 2 2 2 1 1 1 2 1 1 2 1

1

2

2

1 3 3 4 1 1 1 1 3 1 2 2 2

1 1

1

1 1

1 1

1 2 3 4 1 1 1 1 2

1 1 1 2 2 3 1 1 1 1 2

2 2 1 1 1 1 2

1 1 1 1 1 1 1

1 1 1

1 1 1

1 1 1

1 1 1

1

1

1 1 1 1

Chrysophyceae Mallomonas sp. Cyanophyceae Anabaena sp. Chroococcus turgidus (Kütz.) Näg. Gomphosphaeria aponina Kützing Merismopedia tenussima Lemm. Merismopedia glauca (Ehrenberg) Näg. Microcystis flos-aquae (Wittr) Kirschn Oscillatoria sp. Phormidium sp.

1 1

1

1 1

1 1

1 1 1 1 1

2 2 2 1 2

1 1

1 1

1

1

1 1

2 1 1

1

1 1

1

1

2

1

1

1

1

1

2

1

1

2

1

1

1 1 2 1

Dinophyceae Peridinium bipes Stein 4 = high, 3 = moderate, 2 = frequent and 1 = rare abundance.

1 2 2 1

1

1 1 1

1

1

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Table 3. The mean composition of average seawater (Masoud et al., 2001) and mean values for the composition of Lake Qarun water (2001, this study). Parameter

Qarun Lake

Sea water

T. D. S. (g L–1) Total alkalinity (mg L–1) Nitrate-N (mg L–1) Phosphate-P (mg L–1) Calcium (mg L–1) Magnesium (mg L–1) Potassium (mg L–1) Sodium (g L–1) Chloride (g L–1) Sulphate (mg L–1)

34.5 142 0.9 88 374 108 74.2 7.7 15 8500

32.5 206 0.40 38 280 877 308 3.5 17 250

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respectively. Concentrations of sodium were much higher throughout the study period than other cations. Sodium fluctuated from 4.5 g L–1 in January to 11.6 g L–1 in August. Chloride attained a maximum concentration in July (20.3 g L–1) and decreased to a minimum in February (8.2 g L–1). Sulphate concentrations were high throughout the investigation, and the highest level occurred in October (12.9 g L–1) and the minimum in February (5.3 g L–1). Dissolved phosphate-P (SRP) and nitrate-N tended to rise in autumn and decline in summer. Nitrate-N varied from 0.52 mg L–1 in March to 1.5 mg L–1 in November. A minimum value for phosphate-P (0.07 mg L–1 ) occurred in March, whereas a maximum value of 0.12 mg L–1 was reached in November. Total alkalinity increased in summer and decreased in winter, fluctuating between 90 mg L–1 in January and 208 mg L–1 in July. Silicate concentration changed in an inverse way to alkalinity. The maximum value of silicate-Si was recorded in December (220 mg L1) while the 90 mg L–1 minimum value occurred in July. Abundance of phytoplankton There were marked seasonal differences in the quantitative and qualitative composition of phytoplankton communities (Table 2; Figs. 5 and 6). Seasonal variation in phytoplankton abundance was pronounced with a maximum in August (12.6 ¥ 105 cell L–1) and a sharp decline in September; lowest densities occurred in December (0.7 ¥ 105 cell L–1). Changes in total algal counts throughout the investigation coincided closely with Bacillariophyceae abundance. Algal diversity Taxa representing five algal divisions were recorded throughout the investigation: Bacillariophyceae, Chlorophyceae, Cyanophyceae, Chrysophyceae and Dinophyceae (Table 2; Fig. 5). Total percentage composition of the four main phytoplankton groups showed that Bacilla-

Figure 4. Monthly variations in major cations, major anions, chloride, sulphate, nitrate-N, phosphate-P, total alkalinity and silicate-Si in water of Lake Qarun during 2001.

Figure 5. The percentage composition of the main algal groups recorded in the phytoplankton of Lake Qarun during 2001.

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Phytoplankton and water quality in Lake Qarum (Egypt)

Figure 6. Species richness (total number of phytoplankton taxa encounted per standard sample count, see text) and phytoplankton abundance (cell number ¥ 105 L–1) in Lake Qarun during 2001.

Figure 7. Seasonal abundance of common phytoplankon taxa (number of cells ¥ 105 L–1) in Lake Qarun during 2001.

riophyceae dominated the phytoplankton (between 80% in May and 94% in July) throughout the study period. Chlorophyceae ranked second with a maximum monthly average of 17% in January and minimum average of 3% in August. Ranking third were the Cyanophyceae, and were least abundant in summer. Chrysophyceae ranked fourth and Dinophyceae ranked fifth in abundance and were relatively uncommon throughout the year. A total of 49 phytoplankton species were identified during the period of investigation (Table 2). Of these, 16

species were Chlorophyceae, 23 were Bacillariophyceae, 8 were Cyanophyceae, and one each was Chrysophyceae and Dinophyceae. The maximum number of phytoplankton taxa on any one sampling date (40 species) occurred in April, while the minimum (18 species) was in December. The number of phytoplankton taxa are compared with total crop numbers in Figure 6. Phytoplankton species richness was highest in February, but maximum crop density occurred in August (Fig. 7).

Table 4. Pearson correlation matrix of variables measured in Qarun Lake during 2001 (Marked correlations are significant at p < 0.05). Variable Abundance S. R. Temperature pH Transparency Alkalinity Salinity Nitrate-N Phosphate Silicate Sulphate

Abundance S.R. 0.59*

S. R. indicates species richness.

Temp.

pH

Transparency Alkalinity Salinity

0.40 –0.40

–0.20 –0.57 0.23 0.06 –0.82* 0.91* 0.72*

0.73* 0.26 0.80* –0.58* –0.91*

0.44 –0.03 0.84* –0.68* –0.93* 0.84*

Nitrate

Phosphate

Silicate

Sulphate

–0.52 –0.37 0.13 0.22 0.25 –0.42 –0.35

–0.31 –0.28 0.43 –0.53 –0.08 0.03 0.23 –0.03

–0.56 0.08 –0.84* 0.69* 0.94* –0.87* –0.93* 0.45 –0.06

0.07 –0.20 0.84* –0.90* –0.67* 0.58* 0.68* 0.00 0.63* –0.59*

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Figure 8. Seasonal variations in the diversity index (H) of phytoplankton taxa in Lake Qarun during 2001 (see text).

Taxonomic structure of phytoplankton in Lake Qarun showed that the most abundant group was diatoms. Diatoms were dominated by 6 species from a total of 23 taxa. These species were Cyclotella meneghiniana, Cyclotella caspia (= C. choctawhatcheeana), Fragilaria capucina, Fragilaria construens (sensu lato) Nitzschia closterium and Navicula sp. Chlorophyceae were dominated by 2 species from a total of 16 taxa: Chlorococcum humicola and Planktosphaeria gelatinosa. Cyanophyceae were dominated by Microcystis and Phormidium. On the other hand, phytoplankton analyses showed that species diversity was greatest in February (diversity index, 2.88) and lowest (1.75) in September (Fig. 8).

Discussion Lakes in arid zones are poorly studied, yet they are likely to develop and respond to environmental change in substantially different ways from well studied lake systems in northern boreal zones of Europe. Extremely low local precipitation, high salinity and agriculture dominated drainage inflows all combine to make lakes in dry climates especially sensitive and vulnerable systems.

Water quality The water quality of Lake Qarun varied seasonally and suspended material (cf. transparency), pH, salinity, alkalinity, nutrients, and sulphates all changed during the year of study (2001). Photosynthetic algal activity is probably not a major factor affecting lake water characteristics such as pH (Kobbia et al., 1992; Kebede, 1996; Fathi et al., 2001). In summer (June, July, August), transparency and pH were lowest but dissolved oxygen and salinity were highest. Transparency was only ~5 cm during these three summer months but phytoplankton standing crops reached peak biomass in August. Lake water transparency can be affected by phytoplankton and zooplankton blooms (Kobbia et al., 1992; Saleh et al., 2000; Mansour and Sidky, 2003), but suspended sediment is

probably a key factor controlling light availability in Lake Qarun. However, a combination of self-shading, resuspended sediments and silt input probably all contributed to the severe light attenuation in summer. Nutrients do not appear to be limiting; they were not strongly depleted during spring or summer. Inflow of silty drainage water to the lake (data supplied by the Irrigation Department, El-Fayoum) was lowest in February but was high in late spring and summer (March - September), and could be a contributory factor influencing summer water turbidity. The high turbidity was recognized earlier and Girgis (1980) suggested that the turbidity of Lake Qarun was caused mainly by discharging agriculture drainage water. This explanation is only partly supported by the 2001 data where annual drainage inflow is ~395 million cubic meters (data supplied by the Irrigation Department, El-Fayoum) and minimum inflows (February) do not coincide with low periods in turbidity. During spring the weather is known to be particularly windy, and we suspect that wave induced lake sediment resuspension is a significant factor influencing suspended sediments in the lake during this period. Without additional data on local wind speeds and suspended sediments inflows, it is not possible to evaluate the causes of high turbidity. More integrated studies on sediment dynamics are clearly warranted for Lake Qarun. Seasonal changes in lake water salinity show a summer increase as temperature increased. All the soluble salts increased and this is probably caused by evaporation as modified by changes in the inflow regime. Hence, water sampling points furthest from inflows were hypersaline in August 2001, with a maximum value of 43.8 g L–1 being recorded. Turbidity (Secchi disc depth) was negatively correlated with temperature and salinity (P= >0.95, r = 0.91 and 0.93, respectively). Salinity reached high values of 30.9 and 45.3‰ during 1971 and 1994–5, respectively (Meshal, 1973 and Anon, 1997). Soliman (1989) stated that the rate of increase in total salt is expected to remain constant until 2050, while average salinity of the lake may show a progressive increase with time, which eventually must lead

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to the loss of most lake fauna. El-Sayed and Guindy (1999) suggested that the salinity of Lake Qarun rapidly increased since 1960 as a result of inflow diversion of about 60 % of El-Wadi Drain flow to Wadi El-Rayan Lake (a new man-made lake, 40 km, southwest of Lake Qarun). Furthermore, commercial salt extraction from the lake since the 1990s has tended to curb the increasing salt content of lake water (Mansour and Sidky, 2003). The current high salinity is a clear threat to most biota in the lake and high salinity values measured in 2001 are part of a salinization trend that has been apparent for the lake since the 1900s (e.g. Ball, 1939; Boraey, 1980). Salinity is a function of monovalent and divalent cations and these play a role in the productivity of inland waters (Mohammed and Fathi, 1990). The present study shows that Lake Qarun has a high calcium concentration and that this is 2/3 times higher than that of magnesium (Fig. 4). Despite the roles of these ions in algal growth and photosynthesis, there are only a few instances of magnesium deficiency or toxicity in lakes (Goldman, 1960). Magnesium is usually present in aquatic system in large amounts relative to plant needs and is almost certainly not limiting in Lake Qarun. Sodium and potassium also influence the productivity of water (Goldman and Horne, 1983). The monovalent cation content of Lake Qarun showed fluctuating trends throughout the period of study. Talling and Talling (1965) suggested that the amounts of sodium, calcium, and chloride determine the species present rather than the quantitative development of phytoplankton. Although the standing crop of Lake Qarun reached a maximum in summer when the chloride content was at its highest level there is unlikely to be any direct linkage between this ion and phytoplankton growth. The high level of sulphate recorded in 2001 indicates an additional source of sulphate to the lake. In Lake Qarun water, the occurrence of sulphate ions is in the form of sodium sulphate, calcium sulphate and magnesium sulphate. The main source of sulphate ions is probably groundwater and the relatively high sulphate concentration accounts for the major difference between modern Lake Qarun water and seawater. We speculate that groundwater inflows into the lake, through fractures and faults along its bottom, and the dissolution of gypsum are the main source of much of the dissolved sulphate in the lake water (see also El-Sayed and Guindy, 1999; Saleh et al., 2000). The period of elevated alkalinity in the lake during summer correlated positively with higher phytoplankton density (r = 0.73, P = > 0.95). Although carbon dioxide concentration is at a minimum at pH > 8.0 (Kobbia et al., 1992), there seems to be no evidence that this inhibits growth or photosynthetic activities in Lake Qarun. Table 5 shows significant relationships between alkalinity and cell number, pH, transparency and salinity. The lake can be designated as a hard water system (alkalinity is

Phytoplankton and water quality in Lake Qarum (Egypt)

always > 40 mg L–1). Hard waters are generally more productive than soft waters and a positive correlation between alkalinity and fish production has been reported by some authors (El-Sabrouti, 1990; Shakweer et al., 1993). Dissolved silica has a specific role in diatom growth and adequate silica supply is essential for Bacillariophyceae. The observed fluctuations in dissolved silicate concentrations in Lake Qarun were strongly negatively correlated with water temperature and salinity (P = > 0.95, r = 0.84 and –0.93, respectively) and were probably related to variation in silicate uptake by diatoms (Stefansson, 1968). Dissolved silica is supplied to the lake by drainage water (Kobbia et al.,1992; Gad, 1992) and is also generated by remineralization within the lake, although the relative importance of these processes is not yet known. Dissolved silica appeared to be partially depleted during the summer of 2001 but other nutrients were not. Seasonal variation in phosphate and nitrate appeared to be governed by a variety of factors, such as agricultural returns, industrial salt extraction and within-lake processes. Within the lake, increased phytoplankton uptake/ decay and excretion by aquatic organisms can be important (Mohammed and Fathi, 1990; Fathi et al., 2001). Phosphate-P concentration has a highly significant effect on standing crops of Bacillariophyceae in Egyptian waters (Kobbia et al., 1992) as elsewhere (Gibson and Stewart, 1993). Higher values of nitrate-N recorded in this investigation from October to February probably reflected the direct influence of agricultural runoff at this time (Kobbia et al., 1992; Masoud et al., 2001; Mansour and Sidky, 2003). Lower values of nitrate-N occurred during enhanced growth of phytoplankton from March to August (see Fig. 4), but neither N or P appeared to be strongly depleted during the late summer phytoplankton peak. Compared with eutrophic lakes in the northern temperate zone (e.g. Lough Neagh in Northern Ireland, Gibson and Stewart, 1993), mean N-NO3 concentration in Lake Qarun is fairly high as is mean P-PO4 concentration (88 mg L–1). The dissolved N:P (molar) ratio is ~23 and, although total N and P were not measured, low dissolved N:P ratios in culture work (Rhee, 1978) and in multi-lake studies (Smith, 1983) indicate nitrogen limitation and a predominance of blue-green algae. The relatively high absolute concentration of both nutrients in Lake Qarun suggests that neither is limiting. In view of these nutrient values and the large amounts of fertilizers used in the ElFayoum, it is surprising that phytoplankton crop sizes were not larger and that more typical eutrophic algal species were more common. Because of the sampling interval (one month), it is likely that the highest peak values in most variables were missed in 2001 but, even so, light limitation by suspended solids is suspected of suppressing algal growth (see below).

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Phytoplankton The phytoplankton species in Lake Qarun are partly exogenous. The community has several sources, the first and most prevalent is of freshwater origin and the other is saline or brackish. Although the lake is now strongly saline, many species are recognized to be freshwater (e.g. bluegreens, Scenedesmus, Ankistrodesmis spp.). Some species must enter the lake in the drainage water (e.g. Aulocoseira granulata), coming from the Nile (AbdelMoniem, 2001) and some appear to proliferate within the lake and indicate brackish conditions (e.g. Cyclotella caspia/choctawhatcheeana). A major phytoplankton crop peak in Lake Qarun occurred in August and was attributed mainly to the growth of diatoms, especially those in the genus Cyclotella. This peak is rather different from the situation in some temperate regions where spring diatom crops are usually the largest and July-August crops are often green, blue-green algae (Bailey-Watts et al., 1987). Furthermore, changes in phytoplankton abundance were relatively small with a six-fold increase occurring in cell numbers between March and August. In eutrophic temperate lakes, the change in cell abundance of just a single species can exceed two orders of magnitude during the spring bloom (e.g. Asterionella in Scottish Loch Leven in JanuaryMarch 1981; Bailey-Watts, 1988). Periodicity in crop size seems rather muted in Lake Qarun in comparison with northern lakes but not as subdued as in tropical African lakes such as Lake George (Uganda). Despite its relative seasonal consistency, Lake George interestingly also exhibited highest phytoplankton cell numbers in August (1969) (Ganf and Viner, 1973). As far as can be ascertained for Lake Qarun in 2001, crop periodicity seems unrelated to nutrient availability in spring. Suspended solids seem to regulate the phytplankton by suppressing light availability. Sunlight is strongest in July and August and, combined with likely water column stratification, may encourage Cyclotella crops at this time. The diversity of phytoplankton is not high in Lake Qarun and during the 2001 survey a total of 49 species were identified, and these generally agree with those of Abdel-Malek and Ishak (1979). The latter described the phytoplankton community of Lake Qarun as consisting of diverse populations of diatoms with fewer green and blue-green algae (also see Kobbia et al., 1992; Sabae and Rabeh, 2000). According to Kobbia (1992), however, the blue-green algae dominate during summer, but without regular interannual monitoring it is difficult to assess the significance of this observation (and similarly that of the Cyclotella August crop in 2001). Our results and those of El-Awamry (1984) and Abdel-Moniem (2001) for Lake Qarun, nevertheless, indicate that, from the quantitative seasonal estimates, the late summer development of diatom phytoplankton biomass is the most important crop in this lake.

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There are several biological indices of species diversity, based mainly on the composition of phytoplankton (Pielou, 1966; Nygaard, 1978), that can be used to indicate the pollution state of water. In Egyptian waters, several numerical attempts to express degrees of oligotrophy and eutrophy from a consideration of species complements have been made (Kobbia et al., 1992; Sabae and Rabeh, 2000) rather than using nutrient concentrations (Shaaban et al., 1985; Kobbia et al., 1992). Some workers (Soliman, 1990; Fathi et al., 2001; Saleh et al., 2000; Mansour and Sidky, 2003) believe that the biological estimation of the degree of eutrophication and pollution of aquatic ecosystems is probably more informative than chemical indicators (cf. the European Union Water Framework Directive). According to the phytoplankton species present, the water of Lake Qarun tends towards eutrophy. The scale of Staub et al. (1970) to Lake Qarun indicates slight (diversity indexes: 1.0–3.0) moderate pollution depending on the time of year. One problem with the species approach is that some diatom taxa in plankton samples probably originate from the re-working of sediments. Taxa such as Stephanodiscus sp. and Cyclotella ocellata (common diatomite deposits) are almost certainly not currently living in the saline lake today and therefore diversity is over-estimated. Environmental factors other than pollution can also affect the structure of phytoplankton communities, resulting in associations with low diversities. In this respect, El-Shabrawy and Taha (1999) stated that zooplankton in Lake Qarun play a major role in regulation of phytoplankton biomass in summer, when Brachionus plicatilis was the most predominant zooplanker that feeds mainly on diatoms. However, zooplankton grazing is probably not an important regulator of the phytoplankton assemblage in the lake in 2001.

Hydrology This was not assessed directly during the 2001 study. However, it is clear that the inflow drains, carrying silt, nutrients, salts and plankton as well as freshwater from the River Nile, have major influences on Lake Qarun water quality. The influx of water from all the drains reached highest values during late summer and in 1999 the highest inflow occurred in September. These flows are regulated by sluices, although the overall quantity of water available depends ultimately on the level of the Nile River and water released from the Aswan High Dam. Minimum drain inflow values occur in January-February when the irrigation system of the Fayoum is shut-down for cleaning and dredging. This happens every year in this region of Egypt. Hydrology management and variation in the annual water budget clearly deserve further investigation by measuring the fluxes of materials passing to the lake. The salinity of Lake Qarun particularly depends

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upon the fine hydrological balance between freshwater inflow rates and evaporation. This balance is controlled by a variety of interacting environmental factors including climate (especially temperature and wind) and landuse/water supply management practices for crop irrigation within and beyond the Fayoum.

Conclusions Lake Qarun remains a valuable natural and cultural resource despite its salinization during the 20th century. Salinity increase has occurred mainly through changes in water management and freshwater water regulation is recognized as major influence on the biology of North African lakes generally (Flower 2001). The current state of Lake Qarun is strongly influenced by drainage water supply and by high suspended solid loading, influenced in part by strong winds and by the inflow regime. These factors are all thought to play a major role in regulating the aquatic ecosystem of Lake Qarun and, in particular, phytoplankton crops. Freshwater availability to Lake Qarun is locally under human control and wise management is required to ensure the persistence of Lake Qarun as a valuable hyposaline ecosystem. However, in the long term freshwater availability is under climate control and climate change poses a major threat to freshwater supplies in dry regions everywhere. Sustained integrated monitoring of both the inflows and the open lake water for the purpose of modeling future changes is strongly recommended as an aid to formulating wise management policy for Lake Qarun.

Acknowledgments We are grateful to Prof. Dr. I. Kobbia, Professor of Phycology Botany Department, Faculty of Science, Cairo University and Dr. F.T. Zaki , assistant professor of Phycology, Botany Department, Faculty of Science, Cairo University for their assistance and fruitful discussions. Ewan Shilland kindly improved the manuscript text. Our work on this lake was initiated through an earlier EU funded programme (the CASSARINA Project) and the equipment legacy of this programme made the present study possible.

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